The present invention pertains generally to a method and apparatus for surface modification and in particular to a method and apparatus for spraying a surface.
Surface-property modifications obtained by spraying a surface include alloying the crystalline surface layer to improve chemical and mechanical properties, alloying the crystalline surface layer to produce an amorphous surface layer by a rapid self-quenching of the bulk crystalline metal, injecting powder particles of limited solubility into a melted surface layer to produce a dispersion of particles in the surface which improves the abrasion resistance of the material, and applying a molten powder to an unmelted surface to produce a coating with a chemistry and properties distinctly different from the bulk metal.
Alloying the crystalline surface layer is accomplished by a chemical diffusion at an elevated temperature or a melt of the surface layer. Examples of chemical diffusion are nitridation, carburization, and aluminidation. The disadvantages of both chemical diffusion and melting are a possible degradation of bulk properties due to subjecting the entire sample to a high temperature for a prolonged time, a slow production, and a high energy requirement.
The second type of modification has been achieved by weld cladding, laser surface cladding and laser surface glazing. Weld cladding comprises adding material from a consumable welding rod to a surface, thus coating the surface with the welding rod material, and is only suited for production of thick coatings. Laser surface cladding proceeds by applying a powder to a surface and then melting it by laser light. It is essentially a two-step process and suffers from the disadvantages of all multi-step processes. Laser surface glazing modifies a surface by melting and rapidly quenching a metal with a translating laser beam. This process is suited for the formation of an amorphous structure when the bulk chemistry is appropriate.
Injecting powder particles of limited solubility into a melted surface layer as a means for improving abrasion resistance has not been successful. Failures result from present techniques because they have a lower energy density at the surface and require excessive heating times to melt the surface. The generalized and long heating causes too much of the surface to melt for an even distribution of injected particles.
The application of a molten powder to an unmelted surface to produce a coating is generally achieved by plasma spraying which comprises propelling particles to a surface by means of a high-temperature plasma generated by R.F. excitation, the particles being melted by the plasma and impinging on the surface to form a coating. One example of plasma spraying is disclosed in U.S. Pat. No. 3,872,279 to Thomas E. Fairbain whereby a R.F. energy stream heats a powder as it is propelled towards a surface by a stream of inert gas. Surrounding the R.F. energy stream is an annular ring of inert gas which limits the scattering of the beam. A laser beam is focused down this ring of gas and is used to heat the substrate. Flame spraying is sometimes used and it proceeds by introducing a powder into an oxygen-combustible gas torch which fuses and propels the powder to a surface. The major disadvantages of coatings obtained by flame spraying or plasma spraying are the porosity of the coatings, the weakness of the bond due to a lack of wetting of the surface, entrapment of gases in the coating, reaction of the carrier gas with the powder and substrate, and contamination of the coating with material eroded from the electrode producing the plasma.